1. Technical Field
The present invention relates a battery pack, and relates to a battery pack provided with a protection circuit which detects an overcharge, an overdischarge or an overcurrent of a secondary battery and turns off a switch device provided between the secondary battery and a load or a charging unit.
2. Description of the Related Art
In recent years, a lithium-ion battery acting as a secondary battery is mounted in a portable device such as a digital camera. A lithium-ion battery has a low tolerance to an overcharge and an overdischarge. Therefore, a lithium-ion battery is used in a form of a battery pack provided with a protection circuit against an overcharge and an overdischarge.
FIGS. 4 and 5 depict respective examples of a block diagram of a battery pack in the related art. An example of FIG. 4 will be described first. In the example of FIG. 4, a series circuit of a resistor R1 and a capacitor C1 is connected in parallel with a lithium-ion battery 2. A positive electrode of the lithium-ion battery 2 is connected to an external terminal 3 of the battery pack 1. A negative electrode of the lithium-ion battery is connected to an external terminal 4 of the battery pack 1 via n-channel MOS (Metal-Oxide Semiconductor) transistors M1 and M2 which are used to break an electric current.
Drains of the MOS transistors M1 and M2 are connected together. A source of the MOS transistor M1 is connected to the negative electrode of the lithium-ion battery 2. A source of the MOS transistor M2 is connected to the external terminal 4. Further, between drains and sources of the MOS transistors M1 and M2, body diodes D1 and D2 are connected, respectively, equivalently.
A protection IC (i.e., an integrated circuit) 5 has a built-in overcharge detecting circuit, overdischarge detecting circuit and overcurrent detecting circuit. Further, power supply Vdd is provided from a positive electrode of the lithium-ion battery 2 to the protection IC 5 via the resistor R1. Power supply Vss is provided from the negative electrode of the lithium-ion battery 2 to the protection IC 5. Therewith, the protection IC 5 operates.
When an overdischarge or an overcurrent is detected by the overdischarge detecting circuit or the overcurrent detecting circuit in the protection IC 5, a DOUT output of the protection IC 5 comes to have a low level which turns off the MOS transistor M1. When an overcharge is detected by the overcharge detecting circuit in the protection IC 5, a COUT output of the protection IC 5 comes to have a low level which turns off the MOS transistor M2.
Next, an example depicted in FIG. 5 will be described. In the example of FIG. 5, a thermistor R3 is provided in a battery pack 1. One end of the thermistor R3 is connected to a terminal 6 of the battery pack 1. The other end of the thermistor R3 is connected to an external terminal 4. When charging is carried out, a predetermined voltage is applied to the terminal 6 of the battery pack 1 from a charging unit via a voltage dividing resistor. A resistance value of the thermistor R3 changes as a temperature of the battery pack 1 changes. As a result, a voltage at the terminal 6 changes accordingly. The charging unit carries out control such as to detect a voltage at the terminal 6, and stop charging when the temperature of the battery pack 1 exceeds a predetermined value.
According to Japanese Laid-Open Patent Application No. 2004-152580, a series circuit of a temperature protection device (here, a PTC device) and a diode is connected in series with a secondary battery. Further, in parallel with the series circuit, a diode is connected in a reverse direction. By this circuit configuration, the temperature protection device is prevented from operating even when a high temperature occurs during regular discharging of the secondary battery.
The battery pack depicted in FIG. 4 in the related art has no protection function against temperature of the battery pack. On the other hand, the battery pack depicted in FIG. 5 in the related art has a protection function against temperature of the battery pack. However, the predetermined voltage is applied from the charging unit via the voltage dividing resistor to the battery pack of FIG. 5 in the related art. Therefore, a temperature of the battery pack may not be detected precisely when the predetermined voltage of the charging unit changes or an error occurs in a resistance value of the voltage dividing resistor of the charging unit. As a result, protection of the battery pack against temperature may not be carried out with high accuracy.
The present invention has been devised in consideration of the point, and an object of the present invention is to provide a battery pack in which it is possible to carry out protection of the battery pack against temperature with high accuracy.